Infrared hole burning of ammonium tartrate: How high a barrier can be overcome?

The N-D stretching bands of NH3D+ ions doped into ammonium tartrate are studied by spectral hole burning. The crystal structure is known from diffraction studies and the barriers to ammonium-ion rotation from NMR. The N-D bands cover a wide range of hydrogen-bond strengths, and the widths of the bands and the corresponding holes fit the same correlation with frequency as do the N-D stretches in mixed ammonium-alkali sulfates. The energy imparted to the ammonium ion by the excitation of the N-D stretch is about the same as the energy required to surmount the reorientational barrier. However, the bands can be burned and this further defines the possible mechanism for reorientation. The high barriers make it possible to hole burn to high temperatures-up to 90 K. The apparent activation energy for the reorientation of the ions in the dark is about half of the potential barrier, a characteristic of tunneling reactions. (C) 1998 American Institute of Physics. [S0021-9606(98)02813-X].